Effects of Alkyl Side Chain Length on the Structural Organization and Proton Conductivity of Sulfonated Polyimide Thin Films

This study investigates the impact of alkyl side chain length on the structural organization and proton conductivity of sulfonated polyimide (SPI) thin films. SPIs with different alkyl sulfonated side chain lengths CX (X: number of carbon atoms at the side chain, X = 0, 3, 6, and 10) were synthesized, and the relationship between molecular architecture and proton transport properties was investigated. In all SPIs, the polymer backbone was oriented parallel to the substrate, and the lamellar structures were confirmed, with spacing increasing linearly with side chain length. Water uptake behavior and proton conductivity varied significantly, where the C0 thin film exhibited the highest water uptake and proton conductivity at lower humidity while the C3 thin film achieved higher conductivity under high humidity, reaching 1.8 × 10^–1 S cm^–1 at 298 K and 95% relative humidity (RH) and an activation energy of 0.18 eV at 90% RH. Conversely, extending the alkyl side chain length led to the insolubility in water in C10. As a result, a proton exchange membrane system with high chemical/water stability and high proton conductivity was constructed. These findings provide critical insights into designing advanced proton-conducting materials and optimizing their performance for fuel cells and other electrochemical devices.